In a small bowl, dissolve the sazon in warm water and set aside. Warm 2 tablespoons of olive oil in a skillet over medium heat. Add the onion, bell pepper and jalapeno and saute for 5 minutes. Stir in the rice and garlic. When the garlic begins to brown, add the sazon water, cumin, and season with salt and pepper, to taste. Saute until the aromas blend, about 3 minutes more. Remove from the heat, add 1/2 cup of the tomato sauce and set aside to cool slightly, about 5 minutes. Transfer to a large bowl and add the ground beef. Mix until evenly incorporated.
Stuff the cubanelle peppers with the ground beef mixture, using the end of wooden spoon to help ensure stuffing is tightly packed in. Brush the outside of the peppers with remaining olive oil and sprinkle with salt and pepper, to taste. Pour the remaining tomato sauce into an 11 by 9-inch baking dish and arrange the stuffed peppers in the dish. Bake until the peppers are golden brown and the internal temperature registers 140 degrees F on an instant-read thermometer, about 50 to 60 minutes.